The produced oil from oil fields often contains some water. The amount of water produced can vary from close to nil to an extremely high value, for example close to 99% of the total produced fluids. The amount of water produced is usually expressed as a percentage of the total liquid phase and is referred to as the “water cut”.
If the produced fluids include a high water cut, the demands on transportation and processing systems can be very high in comparison to the quantity of oil produced. It is therefore desirable to separate a substantial proportion of the water from the produced fluids immediately or very soon after production from the well. This process is referred to as bulk water removal.
The removed water must be sufficiently free of oil to allow for disposal. The required purity of the water will depend on the method of disposal. For example, if the water is to be re-injected into an oil reservoir, an oil content of 500-1000 parts per million (ppm) will generally be acceptable. However, if the water is to be discharged into the sea, a final oil content of 25 ppm or less will generally be required for environmental reasons.
Separation of oil and water is traditionally carried out in two or more stages. The first stage usually involves using conventional gravity separators. Gravity separators rely purely on gravity (one “g” force) to separate oil from water. A residence time varying from a few minutes to thirty minutes or more is often needed to achieve the first stage of water separation. The need for such a long residence time results in the separators being very bulky with very large fluid inventories.
The second stage of the separation process is usually carried out using hydrocyclones. Hydrocyclones are well known cyclonic separators with a steep conical shape, which are sometimes used for de-oiling water. Examples are described in GB2263077A and GB1506877. They are sometimes referred to as reverse-flow cyclonic separators because the separated phases move in opposite directions to outlets at opposite ends of the separator. Hydrocyclones can provide excellent separation when the oil content of the oil-water mixture is below about 1000 ppm. However, they cannot efficiently separate oil from water when the oil concentration is too high (for example, above 1%). Hydrocyclones also cause a big pressure loss, often exceeding 3 bar or more, which may be undesirable in an oil production installation.
Corrugated plate separators (CPIs) may also sometimes be used for final polishing of the separated water, prior to discharge.
Another type of cyclonic separator is the uniaxial cyclonic separator, in which the fluids flow in the same direction from an inlet at one end of the device to separate outlet chambers at the opposite end of the device. Examples are described in European patent applications EP0313197A and EP0717818A. However, tests using a uniaxial cyclonic separator have shown that this device can only partially separate oil from water, with the separated water still containing a significant quantity of oil. The oil content varies depending on the type and viscosity of the oil but may reach 2% to 5% of the mixture. This is too high to allow the fluids to be discharged without further treatment.
In our UK patent application No. 0801045.6 we describe a separation system for separating a fluid mixture, which includes a uniaxial cyclonic separator that provides a first separation stage and a reverse flow cyclonic separator that provides a second separation stage. We have found that by combining a uniaxial separator with a reverse flow separator in a two-stage separation system we can remove a substantial portion of the water from an oil-water mixture, where the removed water has a very low oil content. For example, we can remove 40-70% of the water from a 50:50 oil-water mixture, where the removed water has an oil content of 5000 ppm or less.
However, we have found that in certain circumstances it can be very difficult to achieve an oil content of 1000 ppm while still removing an acceptable proportion of the produced water, for example 40-70% of the water content. In this case, either the proportion of water removed will be too low (e.g. below 40%) to make the operation acceptable and economical, or the oil content of the removed water will be too high (above 1000 ppm) for it to be re-injected into a reservoir or delivered to a final water polishing system, prior to being discharged into the sea.
The main factors affecting the purity of the separated water may include one or more of the following:                the water-cut (the produced water as a percentage of the total produced oil-water mixture),        the viscosity of the produced oil,        the chemical composition of the oil,        the operating temperature,        the salinity of the produced water, and        the particle size and size distribution of oil and water droplets in the mixture.        
There is therefore a need for a system that can separate water from an oil-water mixture and deliver all or part of the separated water with a very low oil content. Preferably, the system should be capable of removing approximately 40-70% of the water from an oil-water mixture, where the removed water has an oil content of 1000 ppm or less.